The architecture of the physid musculature of Physa
acuta Draparnaud, 1805 (Gastropoda: Physidae).

Abstract:

The basommatophoran family Physidae possesses a remarkable but
little known set of muscles called the "physid musculature".
Using Physa acuta as a model, this musculature was shown to be
anatomically complex and associated in places with the columellar
muscle. The physid musculature has two main components, the physid
muscle sensu stricto and the fan muscle, both of which have previously
been named but not examined in detail. The physid muscle s.s. is
branched with the larger branches running to the neck, head and foot,
and the smaller ones to the lung floor and mantle. The fan muscle is not
branched. We propose that the physid musculature is responsible for a
unique ability of physids to rapidly flick their shells from side to
side--a reaction that frequently enables them to escape predation. We
suggest that during this movement the lung floor, which is strengthened
by several bands of muscle from both the physid musculature and the
columellar muscle, serves as a pivot for the rotating visceral hump and
shell, while the main trunk of the physid muscle s.s. and its main
branches provide a broad anchorage in the foot.

Members of the freshwater pulmonate family Physidae possess a
complex of muscles that is unique amongst gastropods. This complex was
given the name "physid musculature" by Harry and Hubendick
(1964) who provided a brief description of its structure and named its
two main components, the physid muscle sensu stricto and the fan muscle.
Harry (1964) used the presence of the physid musculature as the main
character separating the Physidae from the rest of the Basommatophora.
Paraense (1986, 1987) identified the attachment (insertion) of the
"physid muscle" in his re-descriptions of Physa marmorata
Guilding, 1828 and Physa cubensis Pfeiffer, 1839 respectively. He did
not comment further on this beyond noting that the roof of the pulmonary
cavity was darkly but patchily pigmented except for that part covering
its attachment which in the case of P. marmorata was lightly pigmented
or unpigmented and in the case of P. cubensis, always unpigmented. As
far as we know, no detailed studies have been made of the physid
musculature.

Our attention was drawn to Harry and Hubendick's (1964)
description of this musculature as we looked for a mechanism to account
for the well-known ability of physids to twist their shells rapidly
through approximately 120[degrees] (180[degrees] according to Dawson
1911) in a clockwise direction and back. Dawson (1911) must in fact be
credited with the hypothesis that this twisting is important in predator
avoidance. He indirectly intimated this over 95 years ago from his
observations on the sensitivity of the mantle of Physa gyrina Say, 1821
to external stimuli. He did not record the existence of the physid
musculature, but rather argued that the strong shell-twisting reaction
of Physa gyrina to a localized mechanical stimulus might be due to the
reflection of the mantle over the shell.

This manoeuvre is only seen in physids and is widely thought to
serve as part of predator escape behaviours, e.g. Wrede (1927),
Frieswijk (1957) and in particular, the elaborate avoidance responses
3-6 of Wilken and Appleton (1991). Observations on the
predator-avoidance behaviour of invasive physids in Africa (Hofkin et
al. 1991; Wilken & Appleton 1991; Maharaj et al. 1992; Appleton et
al. 1993, 2004) confirm that these snails are able to escape slow-moving
predators that hunt by ambushing their prey, viz. indigenous leeches and
sciomyzid fly larvae as well as introduced crayfish, but not the
fast-moving indigenous belostomatid bugs that actively pursue their
prey. We therefore support Taylor (2003) in proposing that the physid
musculature plays an important role in the ability of these snails to
avoid predation. This ability to escape both indigenous and introduced
predators has undoubtedly been a useful attribute in the colonization of
many parts of Africa by Physa acuta Draparnaud, 1805 and Aplexa
marmorata (Guilding, 1828) (= Physa marmorata Guilding, 1828) (Brown
1994).

In South Africa, P. acuta has colonized several major river systems
and many smaller ones (de Kock & Wolmarans 2007). It occurs in a
wide variety of habitats, particularly perennial rivers, streams and
dams with muddy or stony substrata, from the coastal lowlands to an
altitude of approximately 1500 m above sea level. In many of these
habitats, it is the most common gastropod species present. A. marmorata
has also become invasive in South Africa, but is found almost
exclusively in standing water bodies notably swamps and ponds, both
natural and artificial (Appleton & Dana 2005). Both species are
still spreading.

Using P. acuta as a representative of the Physidae, this study
provides a detailed description of the physid musculature and proposes
that it plays the major role in shell-twisting. These specimens were
previously known as P. cubensis Pfeiffer, 1839, but this species was
placed into synonymy with P. acuta by Paraense and Pointier (2003).
Although the type locality of P. acuta is the Garonne River, southern
France, it is thought to have been translocated to Europe from the
Americas (Dillon et al. 2001) sometime before 1805.

MATERIAL AND METHODS

Samples of adult P. acuta from several localities in Mexico and
Brazil were relaxed in Petri dishes using two methods. (1) Snails were
placed in fresh water with small pieces of tobacco for five to six hours
at room temperature and then at 4[degrees]C in a refrigerator until they
showed no signs of movement; they were then placed in 70 % ethanol. (2)
Snails were kept overnight in a 0.05% Nembutal solution after which they
were placed in water at 70[degrees]C for [+ or -] 40 seconds in order to
kill them; the soft parts were then pulled from their shells and placed
in a modified Railliet-Henry solution (Paraense 1986, 1987).

Although both methods were effective, the physid musculature proved
easier to dissect and follow in snails relaxed using method 1.
Nevertheless, some variation was seen in the degree of relaxation of
certain muscle bands which resulted in slight differences in their
appearance, especially their width (e.g. Fig. 1C).

Written records were kept as dissections proceeded and drawings
were made at different stages using a camera lucida. These drawings of
the dissected muscles were redrawn within outlines of Physa taken from
Taylor (1988) while referring to actual specimens at the same time.
Finally, composite drawings were made in order to show the whole complex
of muscles and how it associates with the columellar muscle. The terms
'left' and 'right' are used in relation to the snail
body.

A list of abbreviations used in the figures and text is given
below. 'P' denotes a component of the physid musculature and
'C' a component of the columellar muscle.

Pm--main trunk of the physid muscle s.s.

Pm1-5--branches of the physid muscle s.s.

Plu--physid muscle fibres on lung floor

Pp--pneumostome-mantle band

Pf--fan muscle

Cm--main trunk of the columellar muscle

Cm1-4--branches of the columellar muscle

RESULTS AND DISCUSSION

Rather than devise new names for the components of this complex of
muscles, we have followed the terminology of Harry and Hubendick (1964)
with new names given only to components they did not recognize. Thus, we
accept the names "physid musculature" for the entire complex
and 'physid muscle s.s.' and 'fan muscle' for its
two main components. However, to avoid confusion between the
'physid musculature' as a complex and its major component, the
'physid muscle s.s.', these names are written in full or
abbreviated as listed above each time they are used. The physid
musculature of P. acuta is therefore described in terms of its two major
components, their branches and associated structures. Since it is
associated with the physid musculature in several places, the columellar
muscle is described as well.

The physid muscle sensu stricto

The physid muscle s.s. (Pm) is the main trunk of the physid
musculature. It is situated in the right hand part of the body and is
almost as wide as the columellar muscle, the principal muscle of most
gastropods. Its origin is in the right hand side of the right anterior
pedal branch of the main trunk of the columellar muscle (Fig. 1A) and
its insertion is, as shown in Fig. 1B, on the lower right hand side of
the mantle (Harry & Hubendick 1964; Paraense 1986, 1987). The point
of insertion is clearly visible as an elongate scar on the mantle,
broadest close to the mantle collar and extending towards the mid-dorsal
line, but tapering as it does so. The main axis of the physid muscle
s.s. lies perpendicular to the foot and at an angle of [+ or -]
120[degrees] to the columellar muscle. Since this study showed that
there were more components to the physid muscle s.s. than identified by
Harry and Hubendick (1964), they are described in terms of their
association with (i) the upper portion of neck and head, (ii) the lung
floor and pneumostome and (iii) the columellar muscle.

Association with the upper portion of neck and head (Figs 1B-D)

The physid muscle s.s. (Pm) has five branches which are designated
Pm1 to Pm5 in the figures. Two of these (Pm1 & 5) branch from the
upper part of the physid muscle s.s. while the remaining three (Pm2-4)
branch from the lower part (Fig. 1B). Pm1-4 pass down the upper portion
of neck of the snail where they entwine to form a meshlike tissue with
fibres coming down from the columellar muscle (Fig. 1C). Two of these
branches (Pm1 & 2) descend further towards the right and left sides
of the body respectively (Fig. 1B). Pm1 then radiates out over the right
side of the head while Pm2 & 3 as well as the fourth branch (Pm4)
run from the right to the left flank, after passing over the neck. From
here Pm3 & 4 sink towards the foot. Just before doing so, they
entwine with fibres running longitudinally within the body wall for
almost the whole length of the body and with fibres of Pm5 coming from
the right to the left but wrapping round behind the columellar muscle
(Fig. 1B) before entering the foot where it is anchored. The middle two
branches (Pm2 & 3) then run anteriorly towards the left side of the
head, spreading out just above the male gonopore and left eye (Figs 1B,
1D) and sink into the spongy tissue of the sole where they are inserted.

[FIGURE 1 OMITTED]

Association with the lung floor and pneumostome (Figs 2A, 2C)

Thin parallel bundles of fibres (Plu) from the main trunk of the
physid muscle s.s. cross laterally over the floor of the lung cavity
towards the left at approximately the level of the anterior corner of
the pneumostome (Fig. 2A). In addition, fibres from the columellar
muscle cross the lung floor but in an antero-posterior direction, i.e.
at right angles to those from the physid muscle s.s. (Fig. 2A). There
are also some fibres running diagonally from right to left over the lung
floor. These fibres reach the mantle collar but their origin was not
seen. It is thus clear that the lung floor is well supplied with muscle
fibres. It also effectively divides the body cavity into two
sub-cavities, the lower of which includes the mouth, buccal mass and
male and female genitalia while the upper contains the visceral hump.
The muscle fibres associated with the lung floor correspond to the
structure in Lymnaea catascopium Say, 1817 that Walter (1969) called the
"transverse membranous mid-body ('cervical
septum')", but which was difficult to separate from the lung
floor in P. acuta.

[FIGURE 2 OMITTED]

The anterior corner of the pneumostome together with the lung floor
are of particular interest because three different bands of muscle
fibres converge there, one from above and two from below. Because of
their proximity to each other, these bands are thought to combine to
play a role in the swinging of the shell (see below). They are (i) the
pneumostome-mantle band of muscle fibres (Pp) crossing from the mantle
roof (Figs 2B, 2C); (ii) fibres on the lung floor coming from the main
trunk of the physid muscle (Plu); and (iii) those coming from the
columellar muscle (Figs 2C, 3A). The diagonal fibres on the lung floor
(Fig. 2A) were not seen in all specimens dissected. The lung floor is
thus strengthened by fibres from both the physid muscle s.s. and
columellar muscle, but mostly the former, giving it the appearance of
thickened scar tissue. This is in contrast to the 'membranous'
structure described for L. catascopium by Walter (1969). Not only does
this reinforced lung floor form the partition between the upper and
lower body cavities (Figs 2A, 3A), but it is thought to have a pivotal
function in shell-twisting as well (see below).

The columellar muscle (Figs 2A, 3B, 3C, 4A)

The columellar muscle originates on the columella and is inserted
in the foot. In P. acuta it comprises three parts, upper, middle and
lower. The upper part has four elements all of which attach to the shell
and are indicated Cm1-4 in Fig. 3C. The right hand of these elements
(Cm1) descends to the right side of the head while the two middle
elements (Cm2 & 3) descend to the foot. For part of their length,
these three elements are united longitudinally to their contiguous
neighbour or neighbours. The left hand element (Cm4) is divided in its
mid-portion to form two sections (Figs 3B, 3C). The uppermost of these
sections attaches to the edge of the mantle below the distal part of the
digestive tract. It then fans out laterally to the left side to mesh
with the connective tissue of the inner lung wall (Fig. 2A), i.e. at the
"angle of the body whorl" behind the rectum and renal duct.
This section of the lung wall, which lies against the columella, thus
consists of connective tissue reinforced by columellar muscle fibres.

The lower part of Cm4 radiates both laterally and dorsally towards
the head and snout as a wide band of fibres (Figs 3B, 4A, 4C). As it
does so, it allows the passage of the vas deferens and the female and
male gonopores (Fig. 4A). As they pass towards the head, some Cm4 fibres
entwine with those of the physid muscle s.s. (Pm) but most lie above it
as it crosses over the upper portion of the neck from the right to the
left side of the body (Figs 1B-D). Within the head the fibres are so
closely associated with the skin, that it is impractical to separate
them either from the dorsum or the flanks or to determine whether they
originate or insert there. In the foot, the main trunk of the columellar
muscle (Cm) runs posteriorly but does not reach the tip of the tail
(Fig. 1A). Anteriorly Cm divides into two thick branches that run
longitudinally towards the head, tapering as they do so. These pedal
branches were referred to as horns by Elves (1961) in his histological
study on Physa fontinalis. In addition, a thin layer of fibres from the
bottom of the main trunk of the columellar muscle runs anteriorly along
the foot floor but does not quite reach the head.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

These three bundles of muscle fibres in the foot (the main
columellar trunk and its two anterior horns) lie on top of the thick,
spongy skin and enclose a cavity in which the buccal mass, nerve ring
and lower genital ducts lie. This cavity becomes shallower as it
approaches the head. The main trunk of the physid muscle s.s. (Pm) is
anchored primarily in the anterior right horn of Cm though some of its
fibres can be seen to entwine with the left horn (Fig. 1A).

Association with the physid musculature (Figs 1B, 1C, 3C)

The posterior branch of the physid muscle s.s. (Pm5) runs behind
the main trunk of the columellar muscle (Cm) (Figs 1B, 1D, 3C) from the
right to the left side of the body, following the Cm into the foot. Some
of the descending Pm5 fibres entwine with fibres descending from the
columellar muscle (Cm4) and also with fibres from an anterior branch of
the trunk of the physid muscle s.s. (Pm4) that pass over the upper part
of the neck from the right to the left (Figs 1A, 1D).

As described by Harry and Hubendick (1964), the fan muscle consists
of a number of thin but broad bundles of fibres radiating from its
origin at the mantle end of the physid muscle s.s. across the roof of
the right half of the mantle cavity (Figs 1D, 2C, 4B). We agree with the
above authors that these fibre bundles have no clear insertions, but end
diffusely in the tissue of the mantle roof. Some of these fan muscle
fibres entwine in a perpendicular fashion with the pneumostome-mantle
band of fibres (Pp) which appears to originate on the mantle collar.
This band runs from the mantle collar (at the anterior corner of the
pneumostome) across the middle of the right hand half of the mantle roof
(Figs 2B, 2C, 4B) from one side to the other and disappears in the
mantle roof in the vicinity of the spermatheca. It is widest near the
anterior corner of the pneumostome but becomes narrower by a factor of 5
as it reaches the spermatheca.

Interaction between the physid musculature and columellar muscle

Figures 4B and 4C show the components of the physid musculature and
columellar muscle together in composite diagrams in order to demonstrate
their complexity and interrelationship. We agree with Taylor (2003) that
the physid musculature enables physid snails to twist their shells
rapidly as a predator escape manoeuvre in the manner described earlier
but no mechanism has been proposed for this action. We therefore present
one below.

Function of the physid musculature

The following attempt to identify the mechanism responsible for
swinging the shell is based on the description of the physid musculature
given above. It is based entirely on dissection and is therefore
speculative. Further research into the mechanics of the physid
musculature may modify this opinion.

We propose that the components of the physid musculature
responsible for swinging the shell are the fan muscle (Pf), the 5th
branch of the physid muscle s.s. (Pm5) and the pneumostome-mantle band
(Pp). The physid musculature as a whole facilitates the swinging
movement by (i) strengthening the lung floor so that it supports the
rotating visceral hump and (ii) providing a broad anchorage in the head
and foot for the contractions of the effector muscles Pf and Pm5 during
rotation. Note that Pp has no attachment to the foot but originates on
another solid structure, the mantle collar. The lung floor is
strengthened by the meshing of fibres from both the physid musculature
(Plu) and the columellar muscle. Anchorage in the foot is provided by
the origin of the physid muscle s.s. (Pm) in the right hand pedal branch
of the columellar muscle and the individual attachments of its branches
(Pm1-5) in the tissues of head and foot.

The fan muscle and pneumostome--mantle band are the only components
of the physid musculature that lie within the visceral hump, the part of
the body that undergoes twisting. When they contract against the Pm with
its branches 1-5 anchored in the head and foot, these two components
will cause the shell + visceral hump to swing in opposite directions.
When the fan muscle and Pm5, which wraps around the columellar muscle
(Cm), contract together, the hump will swing in a clockwise direction
and when the pneumostome-mantle band contracts, the shell + visceral
hump will swing anticlockwise back to its normal position. This twisting
pivots inside the body, at the base of the hump but dorsal to the
foot--probably on the lung floor which as emphasized above is
strengthened by several layers of muscle derived mostly from the physid
musculature.

The effort required for the initial clockwise twisting is supplied
jointly by the contraction of the fan muscle and Pm5 against the
multiple anchorages provided by Pm in the pedal part of the columellar
muscle and branches 1-4 of the Pm, which are buried broadly in the
anterior pedal mass. The anti-clockwise return movement of the hump is
effected by the pneumostome-mantle band (Pp) and will require less
effort since it is returning to a normal or 'resting'
position. It is possible that Pm4 also plays a role in this
anti-clockwise rotation.

The function of the columellar muscle is to control the protraction
and retraction of the snail's head and foot out of and into its
shell. This is very different from the function proposed here for the
physid musculature, which therefore seems to be largely independent of
the columellar muscle in terms of function though there are some
anatomical associations.

CONCLUSIONS

The physid musculature is an elaborate complex of muscles that is
unique to the basommatophoran family Physidae. Its broad main trunk
originates in the pedal (basal) part of the physid muscle on the foot
floor and is inserted as a single element on the roof of the pulmonary
cavity. There are two principal components, the main trunk (the physid
muscle s.s.) with its five branches and the unbranched fan muscle.
Additional minor muscle bands strengthen the floor of the lung cavity.
All these elements of the physid musculature are believed to play roles
in the shell-twisting behaviour that is characteristic of physid snails.

Our conclusion that the physid musculature enables the visceral
hump and shell to swing in an arc of approximately 120[degrees] and back
rests on four premises. These are (i) that the origin of the main trunk
(Pm) of the physid musculature in the pedal part of the columellar
muscle and the attachments of its five branches (Pm1-5) in the tissues
of the head and foot collectively provide a broad anchorage against
which several of its components (Pf, Pm5 and Pp) can contract; (ii) that
the Pm5 and Pf contract together to cause the shell to twist in a
clockwise direction (Pm5 wraps around the columellar muscle so that when
it contracts, it uses the trunk of the columellar muscle as a pivot);
(iii) that contraction of the pneumostome-mantle band (Pp), which
originates on the mantle collar, causes the shell to return to its
resting position; and (iv) that the anterior corner of the pneumostome
and the thick scar-like tissue of the lung floor provide a robust
platform that serves as a base for the twisting forces exerted by these
muscles. Contractions of Pm1-4 seem unlikely to assist the twisting
action provided by Pf and Pm5. However, these branches of Pm do have a
supporting function by broadening the anchorage of the physid
musculature in the pedal mass as noted above. If it can be shown that
Pm4 plays a role in the anti-clockwise return the shell, Pm4 and Pm5
would be antagonistic muscles.

It is clear that the ability of physids to twist rapidly their
shells in response to certain stimuli is at least partly responsible for
some species, notably P. acuta, being able to defeat attacks from
slow-moving predators by preventing them from making adequate contact
with their shells. Indeed the physid musculature may have evolved within
this Neotropical family in response to predation by such predators, i.e.
glossiphoniid leeches, sciomyzid fly larvae and freshwater crayfish.
Although the latter do not occur naturally in Africa, glossiphoniid
leeches and sciomyzids do, and they are important predators of
freshwater pulmonates there (Appleton et al. 2004). The ability to twist
their shells is a pre-adapted escape manoeuvre that may have helped
physids, particularly P. acuta, to become invasive in regions such as
Africa where they have been introduced. Indeed, P. acuta is probably the
most widespread invasive freshwater gastropod in the world.

ACKNOWLEDGEMENTS

We are grateful to W. Lobato Paraense, Silvana Thiengo and Lygia
Correa (Departamento de Malacologia del Instituto Oswaldo Cruz, Rio de
Janeiro, Brazil) for generously making their facilities available for
this study. Edouardo Prado (IOC) prepared first sketches of the
illustrations, and final versions were prepared and annotated by Albino
Luna, Felipe Villegas and Julio Cesar Montero (Instituto de Biologia,
UNAM). Dr W. Lobato Paraense and Prof. Michelle Hamer (South African
National Institute of Biodiversity, Pretoria) made useful comments on
the manuscript. ENG received financial support from DGAPA-UNAM Mexico,
and CCA from the Research Fund of the University of KwaZulu-Natal, South
Africa.